The shooting occurred in the 2500-block of North Lowell Avenue at about 9:14 a.m., police said.

Family identified the victim as Jose Nieves, 38. He was taken to Illinois Masonic Hospital where he was pronounced dead, police said. Police said Nieves was not armed

"The person who was shot did not have a weapon. That much we know. The officer's weapon is the only one we found," said police spokesman Anthony Guglielmi.

At a news conference Monday afternoon, Chicago Police Superintendent Eddie Johnson said the shooting was a result of a verbal altercation and that the officer and the subject knew each other from a prior incident. Johnson and IPRA investigators are trying to determine how that verbal altercation turned deadly.

"These two individuals knew each other from a confrontation a few weeks ago," Johnson said.

The off-duty officer has only been identified as a 57-year-old veteran of the mass transit unit. He has been moved to desk duty for the duration of the investigation into the shooting.

Family said Nieves worked security at a local bar and was with his girlfriend when the confrontation took place. Family and friends said the off-duty officer had lived in the same buildings as Nieves, and they had had several previous encounters.

"He would complain about the guy pulling out his gun at him, him coming home from work. More than once, he's called 911. They've gone to the apartment. They've gone there. They don't do nothing about it. He's an officer," said Angelica Nieves, the victim's sister.